1. Field of the Invention
One embodiment of the present invention relates to a thin film transistor and a manufacturing method thereof and a display device and a manufacturing method thereof. One embodiment of the present invention further relates to an electronic appliance.
2. Description of the Related Art
Recently, a thin film transistor (hereinafter, referred to as a TFT) in which a semiconductor thin film (the thickness is approximately several nanometers to several hundreds of nanometers) is provided over a substrate having an insulating surface (e.g., a glass substrate) has attracted attention. The development of TFTs, for example, as switching elements of a display device such as a liquid crystal display device and the like has been accelerated. As a semiconductor thin film of the TFTs, an amorphous semiconductor film or a polycrystalline semiconductor film is mainly used. There are also TFTs in which a microcrystalline semiconductor film is used (e.g., Patent Document 1). In a display device, switching characteristic of the mounted TFTs has influence on a display quality, power consumption, and the like.
One of the parameters that determine the switching characteristic of the TFT is an On/Off ratio of current. Here, an On/Off ratio of current refers to a ratio of ON current to OFF current. Note that OFF current refers to current flowing between a source and a drain when the TFT is OFF while ON current refers to current flowing between the source and the drain when the TFT is ON. Consequently, in order to increase an On/Off ratio of current, ON current may be increased and OFF current may be decreased.
Note that in this specification, the term a “source” alone refers to either or both of a source electrode and a source region and the two are not particularly distinguished. Similarly, the term a “drain” alone refers to either or both of a drain electrode and a drain region and the two are not particularly distinguished.
Note that in the TFT, a high On/Off ratio is not the only requirement. It is also important to reduce light-induced leakage current, for example. Here, light-induced leakage current refers to unintended current which flows between the source and the drain due to a photovoltaic effect when light reaches a semiconductor thin film in the TFT. Particularly, since a TFT which is used as a pixel transistor of a liquid crystal display device receives light from a backlight on a substrate side, light-induced leakage current should be sufficiently small. Accordingly, there have been a lot of developments in a technique for shielding the semiconductor thin film of the TFT from light (Patent Document 2, for example).